LED lighting offers many potential benefits over incandescent, halogen, fluorescent and gas/arc lamps because, at least, such lighting has a longer life time, no mercury, and a fast evolutionary improvement of efficiency. Those skilled in the art note movement throughout the world to ban or limit the use of incandescent lamps. LED lighting is viewed as being a viable replacement. However, a primary challenge to the exploitation of LED lighting, especially in low power bulbs, is the size and cost of the driver circuitry which must be included in each bulb.
The most commonly used topology for an offline LED driver is the flyback topology. On the primary side, a controller and discrete power devices are necessary for power conversion. In order to have an accurate and constant LED current, driver circuits typically utilize a secondary LED current feedback. An opto-coupler is also required to transfer the sensed signal from the secondary side past the isolation barrier to the primary side controller. For low wattage bulbs, this the secondary-side and feedback components can be a problem since there is little space available in the bulb for including the necessary circuit components.
The solution to this problem is to use primary control. Those skilled in the art understand the primary control topology regulates the LED current through the primary side. One easy way to implement primary control is to provide for constant power control in flyback. A drawback of this solution for constant power control is that the LED current may not be constant when the LED forward voltage varies. Another approach is to estimate the secondary output current feedback by sensing primary peak current and duty cycle through an analog circuit or digital circuit. For example, the LED current may be calculated by sampling the peak value of the primary current and detecting the secondary diode conduction period. There are two main concerns with this solution. First, a high speed sample and hold circuit is required to measure the peak value of the primary current. Second, there is a time delay between the sample and hold to the actual switching which introduces an error in the current measurement. In yet another approach for primary control, the primary current reference is directly used for controlling output current. Advantageously, this eliminates the need for a sample and hold circuit, and a reduction in overall size and cost of the circuit is achieved by integrating a high voltage avalanche-rugged power MOSFET with the controller circuitry. Unfortunately, this solution is available for DC voltage input only.
There is a need in the art for a cost effective power factor correction function implemented by means of a highly integrated driver device so as to minimize the number of external components and provide a compact and cost effective solution.